What is Clamp Direct/Indirect?
Clamp puts a limit to the intensity of light bounced directly and indirectly by surfaces.
It can be used to prevent the annoyoing "fireflies" (random intense pixels in your render) but it is advised to avoid using this feature as it harms the realism of the rendered scene.

It is a good advice to never ever use clamping on direct, and if you're going to use indirect, keep the value rather high (close to the clipping value of the view transform you use) so you don't lose too much energy in your render and bounces and reflections stay realistic.

Using such conservative values for clamping will reduce its efficiency at preventing fireflies, but keep in mind that if you're aiming to photorealism clamping should be your last resort.